Minimizing the beam width or “spot size” of a focused beam of light is important in a number of technology areas. For example, spot size is important for writing data to, and reading data from, an optical disk. The smaller the spot size, the higher the recording density of the optical disk.
In order to obtain a small spot size, a beam is typically passed through a convex lens with a high numerical aperture (“NA”). The NA of a convex lens can be increased by increasing the thickness of the lens in the direction of the optic axis and shortening the focal length. However, incorporating such lenses in optical-based devices may be cost prohibitive because of the difficulty in fabricating very small lenses with large curvatures and because of the precise polishing needed to make the lenses aberration free. In addition, conventional convex and concave lenses may not be compatible with planar integrated circuits.
Focusing grating couplers can also be used to produce a focused beam of light. With a focusing grating coupler, light propagating along an optical waveguide can be projected into free-space outside of the waveguide and focused. Although a focusing grating coupler can be used to couple light from a waveguide to free space, it cannot be configured and operated to couple light from free space to free space.
Although in recent years there have been a number of advances in sub-wavelength optical element design and fabrication, designers and manufacturers of optical elements continue to seek enhancements that broaden the possible range of optical element designs and applications.